Hey everyone, so I don't have the APM tuned very well but at least its hitting waypoints and holding altitude and airspeed. I decided to get it flying and see just how long it would go. Here it the result:
this is the equipment list:
APC 12*8 prop
2x Zippycompact 6s 5000 mah lipo
APM 2.5 + telemetry + airspeed sensor
So whats next for the techpod? well, I will refining the gains. After I have it flying smoothly, I have a setup in the works that will be pushing flight times over 5 hours. Stay updated on the lated techpod news @ hobbyuav.com
Comments
Martin Hepperle and Michael Selig are the undoubted experts - read their work on props - no 'gut-feel' there - large diameter,high advance ratio props are ALWAYS more efficient. But using the prop specs to decide on the systems efficiency is highly flawed. Most, if not all brushless motors, are most efficient at the high end of the design RPM range. ESC's are most efficient at the max PWM throttle input setting. Using a larger diameter, high advance ratio prop, spinning at is optimal speed ( look at its N100 data..) using a low kv motor, will always yield the best efficiency. Launch is however not an efficient regime, and excess pitch will stall the prop , with very poor launch capability.
Take a look at my SurVoyeur blog ( old now, bit still valid, except for the mk-II plane recently maidened. and due on the blog soon). The mk-I in the blog started life on a 700kv hacker motor, with a 12X8 Aeronaut folding prop.
Launch amps are 48, on 4 cell Lipo. Flight time ( endurance only) is 48minutes. CLimb rate is 3.2m/s (5kg AUW)
Replacing the motor with a Hacker 500kv version, still 4 cells, and a 15X12 Aeronaut folding prop, gave 43amps at launch, climb rate of 4.1m/s and 73minutes endurance.
Joe
The Nampilot
I agree, for easy launch capability a large prop wins.
For cruise and typical prop design I still believe a rather small prop will pay.
If you want to go for large props, you probabely have to choose a very narrow design for them to be efficient, maybe even with saw teeth:
http://www.leomotion.com/shop/USER_ARTIKEL_HANDLING_AUFRUF.php?dars...
I'm wondering which has better L/D
when producing the same thrust.
If you add easy hand launch capability to the requirements then I think that a larger prop always wins.
Thanks for the references. Interesting papers!
My conclusions:
The Colorado State University paper says: "Within the constrained design space, optimal powertrain designs were found to use large diameter, slow-turning, high pitch propellers and 2 or 3 cell lithium-polymer batteries." The rule of thumb of pitch ratio around 1 is therefore confirmed. Unfortunately they give no rules of thumb for diameter and rpm (pitch speed). But they contradict my thesis of relatively small diameters.
The Israeli paper says: "It can be concluded, based on the sensitivity study, that the maximum power-to-mass ratio has the largest influence on the design and performance of the system." The importance of maximum power per flight weight is confirmed. They climb around 80W/kg flight weight, which is close to my rule of thumb of 100W/kg. They loiter at around 12W per kg, which seems quite low to me, that must be an efficient airframe.
Combining the two papers I still think my rules of thumb are quite ok.
Any other conclusions concerning rules of thumb for prop (and motor) choice?
And this one also --> www.gordon-se.technion.ac.il/files/2012/05/421072-11.pdf
The conclusions are quite interesting.
This paper might also be of interest --> http://www.engr.colostate.edu/~thb/Publications/SD%20UAV%20Final%20...
Yup, the Fire-Storm is a brilliant design. Once more from the Paparazzi guys :-)
http://paparazzi.enac.fr/wiki/Fire-Storm Not bad for a small plane.
Thanks for your thoughts. I agree there is a difference between optimizing on long endurance (time aloft) and long distance. For long endurance a slightly smaller pitch might pay. But for longest time aloft I recommend using a balloon or a blimp ...
The statement "larger props are more aerodynamically efficient then smaller ones" is not generally true. I have read the paper of D. Rogers and therefore don't like it. The reasons are that with a excessively large prop you get too much drag towards the prop tip and you get too much wetted area. Optimizing props is not quite as simple as optimizing glider wings by increasing span and aspect ratio.
It is simple to build an airplane with a huge prop diameter, e.g. same as its wing span. Why does nobody do it, not even record planes, niether in models nor in full scale? Because its not efficient! The rules of thumb noted above (pitch ratio, pitch speed, power ratio) work better.
Prop development is still ongoing. See for example the developments of LeoMotion. I am interested in how things go on!
Sweet setup:P Just ordered mine:P Ditch a battery and go solarpowered:P
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